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CFDJ: Article

Using Gateways PART 2

The basics

Part 1 in this series (CFDJ, Vol. 7, issue 5) discussed the options in the ColdFusion Administrator for configuring the event gateway facility in ColdFusion MX 7 and how to register new event gateway types.

There was little in the way of explaining how to use event gateways, so in this article we'll explore the basics of using event gateways through two examples: the code from the previous article and another of the out-of-the-box event gateway types, the CFML asynchronous event gateway type. These two gateway types represent the two fundamental varieties: the types that respond to events external to a ColdFusion application and the types that respond to events generated within a ColdFusion application.

Event gateways represent a new kind of service architecture for ColdFusion that is composed of a number of logical groups, each with its own intricacies. This article glosses over much of the deeper details with the intent of getting you up and running without bogging you down. Not to worry though, the finer details ignored now will be covered in greater detail in a later article.

A rough overview of the choreography of gateways can be seen in the ColdFusion MX 7 product documentation (Developing ColdFusion MX Applications, Using Event Gateways, About event gateways, How event gateway applications work). Many of the framed pieces in this diagram are identified with common event gateway jargon discussed in Part 1. [http://livedocs.macromedia.com/coldfusion/7/ htmldocs/images/gatewayb.jpg]

The two discrete pieces we're going to discuss in this article are the listener CFCs and CFM Pages (the calling logic being either a CFML template or a ColdFusion Component).

CFML Code for Gateways
To create a gateway instance you need a gateway type, a configuration file (which is optional in some cases) and a listener CFC. ColdFusion MX 7 comes with seven gateway types to start you off, one of which, the DirectoryWatcher type, was used in the previous article to verify the instructions for creating a gateway instance. Since the sample code was for an event gateway that responded to external OS level events, it had only a listener CFC written for it, which is a good place to begin.

Listener CFCs
Developers familiar with frameworks in other languages will recognize the role that the methods in a listener CFC fulfill - the event handler. Similar to a class implementing a component or control delegate in the .NET Framework or a Java bean event listener in Java, the listener CFC is the chunk of code that is the implementation-specific part of the more generic problem domain as defined by the gateway type.

While the gateway type defines the method names, the signature always remains the same (with the obvious exception of the method name itself): a public method with one argument, a structure, which may return a string. (Note: Signature is a term used to indicate a function's or a method's definition, its name, the names and data types of its arguments, and data type of the returned value [http://en.wikipedia.org/wiki/Signature_%28Computer_Science%29].)

Event Handling Methods
The one method argument is called the CFEvent and contains information about the gateway instance, the default listener CFC that the gateway instance will use, and a structure containing any data elements sent by the method's calling logic. These data elements can be populated by the Java code of the gateway type as well as the CFML code invoking the gateway instance.

The Members of CFEvent
Each event handler method receives a single structure, the CFEvent argument. This structure contains seven members by default that are almost solely for informational purposes. They are listed in Table 1 and in the ColdFusion product documentation (CFML Reference, ColdFusion MX Event Gateway Reference, CFML CFEvent structure).

Reviewing the Previous Sample
Looking at the code from Part 1 (available at: [http://gemsres.com/story/may05/86134/source.html]), we find two files, CCOJASDirectoryWatcher.cfg (the configuration file) and CCOJASDirectoryWatcher.cfc (the listener CFC), which were used to create the gateway instance, CCOJASDirectoryWatcher.

Within this component we see three methods: onAdd, onDelete, and onChange. These methods are defined by the DirectoryWatcher gateway type (Developing ColdFusion MX Applications, Using Event Gateways, Using the example event gateways and gateway applications). These methods are the event handlers for the DirectoryWatcher gateway instance and none of them are required. If a listener CFC does not implement one of these event handling methods, the gateway instance does not attempt to make a call to the method. This means that the listener CFC needs to implement only what is germane to the problem domain and nothing more.

The CFEvent's data Member
In the case of this example, all three event handling methods were implemented and, as was demonstrated in Part 1 ("Cold Cup o' Joe - Another Shot: Gateways to Fun, Putting It All Together"), a log file was updated each time a file was added to, changed, and deleted from the watched directory. What is more interesting now is not just the execution of the gateway instance but the variables that get passed to the event handling methods.

Referring to CCOJASDirectoryWatcher.cfc from Part 1, you'll see that the first functional line of each method (lines 5, 12, and 19) pull out a structure called data from each method's CFEvent argument. This contains data about each OS level event that the DirectoryWatcher was monitoring for. This structure contains three members, identified in the ColdFusion product documentation (Developing ColdFusion MX Applications, Using Event Gateways, Using the example event gateways and gateway applications, Example event gateway, CFC methods) and shown in Table 2.

More Stories By Guy Rish

Guy Rish is a ColdFusion and .NET developer at Vente as well as President at Gestaltech. He is an active developer and writer for various languages and technologies, and has contributed work in books on ColdFusion MX, Flash MX, and Dreamweaver MX. He blogs at ccoj.coldfusionjournal.com.

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